The present invention relates to a fuel/auxiliary oil thermal management system. More particularly, it relates to a fuel/auxiliary oil thermal management system which includes fuel/oil heat exchanger means, temperature sensing means, recirculating flow control means, and ram air heat exchanger means.
Most aircraft have auxiliary oil systems that require cooling to maintain temperatures within allowable limits. These include hydraulic systems, gearbox oil systems such as the engine gearboxes or airframe mounted accessory gearboxes, and generator oil systems such as Integrated Drive Generators (IDG) or Variable Speed Constant Frequency (VSCF) generators. These systems are usually forced-air-cooled, using compact heat exchangers which require freestream (ram) air brought onboard the aircraft with scoops or other type inlets.
The benefit of this type of system is that the supply of such ram air is unlimited. In addition, some quantity of air is required anyway for compartment and engine nacelle ventilation.
The main disadvantage, however, is that ram air creates drag on the aircraft. This translates directly to aircraft weight and therefore more fuel is required to complete the same mission. Another important consideration is that the temperature of ram air varies over the entire flight envelope (ram air temperature rise over static ambient air temperature is proportional to the square of the aircraft Mach number).
FIG. 1 gives some maximum ram air temperatures seen by a variety of military aircraft in a MIL-STD-210A hot atmosphere (39.4.degree. C. or 103.degree. F. at sea level). Usually the flight conditions where these maximum ram air temperatures are seen are the same as those for which auxiliary oil system heat loads are the highest. Thus, ram air flow rates to cool these systems are often significant. Even with large air flow rates, oil systems temperatures in the air-cooled system can reach levels which are only allowable for short transient periods (during a specific maneuver). In addition, during ground static operation, ram air is not readily available and as such, fans or bleed air ejector systems must be provided.
A fuel-cooled system, on the other hand, takes advantage of the heat capacity of the aircraft fuel already onboard. Other advantages are that fuel/oil heat exchangers are smaller and lighter than similarly rated air/oil heat exchangers, less ram air is required and therefore overall aircraft drag is reduced. Auxiliary oil systems temperatures are lower and more uniform since the fuel heat sink is more uniform. During high speed maneuvers when air-cooled systems run hot, the fuel-cooled auxiliary oil system usually decreases in temperature due to high engine fuel flow rates. This lower operating temperature results in increased system reliability. Fuel flow is available during ground static operation as well, so the need for bleed air ejectors or fans is eliminated.
A disadvantage of this type of system is that the cooling capacity of onboard fuel is not unlimited (as with any expendable cooling system), and recirculation tends to increase aircraft tank fuel temperatures over time. Care must be taken, therefore, in the design of a fuel-cooled system so that for the most severe mission, in the hottest design environment, the subsystem cooling capability is not depleted before the fuel itself is depleted.